The present disclosure relates generally to oilfield logging and, more particularly, to oilfield logging involving inelastic gamma-ray measurements.
Many properties of a subterranean formation may be determined using different oilfield logging techniques, which may involve one or more tools having a radioisotope source. For example, to locate gas in a subterranean formation, a conventional practice combines data obtained from two tools. One of the tools is a “density” tool, which measures the electron density of the formation, and the other of the tools is a “neutron porosity” tool, which generally measures the density of hydrogen in the formation, known as the “hydrogen index (HI).” Based on measurements of formation density and hydrogen index, the porosity and pore fluid density of the formation may be determined. For a given formation fluid density, or gas saturation, a combination of a decrease in the formation density and an increase in the hydrogen index indicates an increase in the porosity of the formation. Meanwhile, for a given formation porosity, a combination of a decrease in the formation density and a decrease in hydrogen index indicates a decrease in the pore fluid density and hydrogen content. For pores filled with water and gas or oil and gas, the density and hydrogen index are an indication of the gas saturation (volume fraction of the pores occupied by gas). For pores filled with gas only, the density and hydrogen index are an indication of gas density (pressure).
The density and neutron porosity tools for measuring formation density and hydrogen index may generally employ radioisotope sources to obtain formation density and hydrogen index measurements, respectively. For example, the density tool may use a source such as 137Cs to emit gamma-rays into a formation. Based on a count of gamma-rays scattered by the formation, the density tool may determine the electron density of the formation. Similarly, the neutron porosity tool may use a source such as 241AmBe to emit neutrons into a formation. A count of neutrons scattered by the formation may yield a hydrogen index measurement. Such radioisotope sources may be disadvantageous in oilfield tools, as the sources may be heavily regulated by law and they can be hazardous since they cannot be shut off.
In lieu of such radioisotope sources, an electronic neutron generator may be used which will produce neutrons which, in turn, produce gamma-rays. To do so, the electronic neutron generator may emit neutrons into a formation, which may in turn produce gamma-rays via inelastic scattering and neutron capture events. A count of gamma-rays produced by inelastic scattering may generally yield a signal that is related to formation density, and a count of scattered neutrons may generally yield a neutron porosity signal that corresponds to the hydrogen index of the formation. Alternatively, a count of capture gamma-rays may generally yield a neutron porosity signal that corresponds to the hydrogen index of the formation. If it is not possible to separate the inelastic and capture gamma-rays to produce nearly independent signals sensitive to formation density and hydrogen index, respectively, then the two signals may not be used together to enable a precise determination of porosity and gas saturation.
Neutron reactions that produce gamma-rays may be separated according to the energy of the neutron. After a 14 MeV neutron has been emitted by the source, it begins to lose energy by the processes of elastic and inelastic scattering. Inelastic scattering events are typically produced by neutrons in the energy range 1-14 MeV. After neutrons have decreased in energy below approximately 1 MeV, they typically have insufficient energy to inelastically scatter; however, they continue to lose energy by elastic scattering. The decrease in energy from 14 MeV to 1 MeV happens very rapidly, in a time typically less than 1 microsecond. Inelastic scattering reactions therefore occur very quickly after the neutron leaves the source, typically in less than 1 microsecond. From approximately 1 MeV down to thermal energy (approximately 0.025 eV), neutrons decrease in energy by elastic scattering over a time interval that ranges from 2 to several microseconds, depending on the amount of hydrogen in the formation. During that slowing time, neutrons may be captured and this may lead to the emission of one or more gamma-rays. These are so-called “epithermal” capture gamma-rays. Neutrons which decrease in energy completely to thermal energy continue to elastically scatter at that energy, often for many hundreds of microseconds until they are captured and this may lead to the emission of one or more gamma-rays. These are so-called “thermal” capture gamma-rays. Since neutrons are emitted from an electronic neutron source typically in bursts no shorter than 10 microseconds, it will be appreciated that the inelastic and epithermal capture gamma-rays are emitted substantially within that 10 microsecond burst and therefore overlap in time. Thermal capture gamma-rays, on the other hand, extend into the time interval between bursts as well as during the burst. Since there is overlap of capture and inelastic gamma-ray events during the burst, simply summing over all counts during the burst may yield a signal that corresponds, at least in part, to the hydrogen index of the formation, rather than to formation density. Various techniques to correct for the component of thermal neutron capture gamma-rays have been disclosed, for example, in U.S. Pat. No. 5,374,823 to Odom. Odom suggests that epithermal capture gamma-rays might be corrected if the epithermal neutron lifetime were known but gives no guidance on how to do this and suggests that it is unimportant anyway. Trcka, in U.S. Pat. No. 7,365,308 mentions the problem of epithermal capture gamma-rays but is silent on how to correct for them. In a similar way, Wilson in U.S. Pat. No. 6,207,953 discusses an “inelastic” gamma-ray count rate but this is just the burst sum corrected for thermal capture gamma-rays. No attempt is made to correct for the epithermal gamma-ray contamination. However, unless both thermal and epithermal capture gamma-rays are eliminated from the burst sum, the resulting signal will generally correspond in large part to hydrogen index rather than formation density, and is redundant with a capture gamma-ray or scattered neutron measurement of hydrogen index.